Optical pickup and disk drive unit

ABSTRACT

An optical pickup and disk drive apparatus has a light-emitting device ( 14 ) having a first light emitter ( 14   a ) and a second light emitter ( 14   b ) for emitting a first laser beam and a second laser beam, respectively, a light splitter ( 18 ), a collimator lens ( 19 ) for converting the first laser beam and the second laser beam into a parallel beam, and a light-detecting device ( 29 ). The first laser beam has an optical axis aligned with the optical axis of an objective lens ( 13 ), and the second laser beam has an optical axis displaced off the optical axis of the objective lens. The second light emitter is placed in a position where the angle α of incidence of the second laser beam on the light splitter is greater than the angle θ of incidence of the first laser beam on the light splitter, for thereby reducing the amount of generated aberrations for increased performance.

TECHNICAL FIELD

The present invention relates to a technical field of optical pickup anddisk drive apparatus. More specifically, the present invention relatesto a technical field for reducing an amount of generated aberrations forincreased performance in an optical pickup for recording and/orreproducing information signals on and/or from two different types ofdisk-shaped recording media and disk drives which have such an opticalpickup.

BACKGROUND ART

There are disk drive apparatus for recording information signals on andreproducing information signals from disk-shaped recording media. Such adisk drive apparatus has an optical pickup movable radially across adisk-shaped recording medium mounted on a disk table for applying alaser beam through an objective lens to the disk-shaped recordingmedium.

Some disk drive apparatus are capable of recording information signalson and reproducing information signals from disk-shaped recording mediaof different types, e.g., CD (compact disks) and DVD (digital versatiledisks).

Such disk drive apparatus are required to emit two laser beams havingdifferent wavelength ranges from a light-emitting device because ofdifferent recording densities on CD and DVD. For the purpose of reducingthe size of the optical pickup, a disk drive apparatus has an objectivelens shared by two types of disk-shaped recording media and alight-emitting device comprising two light emitters disposed in onepackage for emitting respective two laser beams having differentwavelength ranges. In such a disk drive apparatus, for example, thefirst light emitter emits a first laser beam having a wavelength of 650nm to a DVD, and the second light emitter emits a second laser beamhaving a wavelength of 780 nm to a CD.

However, since the objective lens of the above disk drive apparatus isshared, the first light emitter and the second light emitter aredisposed parallel to each other in a direction perpendicular to theoptical axis of the objective lens. Therefore, though the optical axisof the laser beam emitted from one of the light emitters can be alignedwith the optical axis of the objective lens, the optical axis of thelaser beam emitted from the other light emitter has to be off theoptical axis of the objective lens.

Since the optical axis of the laser beam emitted from either one of thelight emitters needs to be off the optical axis of the objective lens,when the laser beam is emitted from the light emitter whose optical axisis off the optical axis of the objective lens, an amount of aberrationsthat are generated tends to be large, resulting in a reduction in theperformance of the optical pickup thereby to cause a recording error anda reproduction error due to a signal level reduction or the like.

An optical pickup and a disk drive apparatus according to the presentinvention are aimed at overcoming the above problems and reducing anamount of aberrations generated for increased performance.

DISCLOSURE OF INVENTION

To solve the above problems, an optical pickup and disk drive apparatusaccording to the present invention has a light-emitting device having afirst light emitter and a second light emitter for emitting, to twodifferent types of disk-shaped recording media, respectively, a firstlaser beam and a second laser beam, respectively, having a firstwavelength and a second wavelength, respectively, which correspond tothe respective disk-shaped recording media, a light splitter for passingthe first laser beam and the second laser beam which are emitted fromthe light-emitting device toward the disk-shaped recording media, andreflecting the first laser beam and the second laser beam which arereflected respectively by the disk-shaped recording media, a collimatorlens disposed between the light splitter and the objective lens, forconverting the first laser beam and the second laser beam which areemitted from the light-emitting device into a parallel beam, and alight-detecting device for being irradiated with the first laser beamand the second laser beam which are reflected by the light splitter, thefirst laser beam having an optical axis aligned with the optical axis ofthe objective lens, the second laser beam having an optical axisdisplaced off the optical axis of the objective lens, wherein the secondlight emitter is placed at a position where the angle of incidence onthe light splitter of the second laser beam emitted from thelight-emitting device toward the corresponding disk-shaped recordingmedium is greater than the angle of incidence on the light splitter ofthe first laser beam emitted from the light-emitting device toward thecorresponding disk-shaped recording medium.

Another optical pickup and disk drive apparatus according to the presentinvention has a light-emitting device having a first light emitter and asecond light emitter for emitting, to two different types of disk-shapedrecording media, respectively, a first laser beam and a second laserbeam, respectively, having a first wavelength and a second wavelength,respectively, which correspond to the respective disk-shaped recordingmedia, a light splitter for passing the first laser beam and the secondlaser beam which are emitted from the light-emitting device toward thedisk-shaped recording media, and reflecting the first laser beam and thesecond laser beam which are reflected respectively by the disk-shapedrecording media, a collimator lens disposed between the light splitterand the objective lens, for converting the first laser beam and thesecond laser beam which are emitted from the light-emitting device intoa parallel beam, a light-detecting device for being irradiated with thefirst laser beam and the second laser beam which are reflected by thedisk-shaped recording media, and a coupling lens disposed between thelight-emitting device and the collimator lens, for changing an opticalmagnification from the light-emitting device to the disk-shapedrecording media and an optical magnification from the disk-shapedrecording media to the light-detecting device, the first laser beamhaving an optical axis aligned with the optical axis of the objectivelens, the second laser beam having an optical axis displaced off theoptical axis of the objective lens, wherein the distance from thelight-emitting device to the coupling lens is of 0.26F or more where Frepresents the combined focal length of the collimator lens and thecoupling lens.

The optical pickup and disk drive apparatus according to the presentinvention is thus effective to reduce the amount of aberrationsgenerated when the second laser beam is emitted.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1, together with FIGS. 2 through 8, shows an embodiment of thepresent invention, FIG. 1 being a schematic perspective view of a diskdrive apparatus;

FIG. 2 is a conceptual diagram showing an arrangement of an opticalsystem in an optical pickup;

FIG. 3 is a conceptual diagram showing angles of incidence of laserbeams applied to a light splitter and the relationship between a focallength F and a distance D;

FIG. 4, together with FIGS. 5 through 8, shows amounts of aberrationsgenerated with respect to the inter-emitter distance at the time thedistance D is changed with respect to the focal length F, FIG. 4 being agraph plotted when D=0.14 F;

FIG. 5 is graph plotted when D=0.26F;

FIG. 6 is graph plotted when D=0.38F;

FIG. 7 is graph plotted when D=0.51F; and

FIG. 8 is graph plotted when D=0.64F.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of an optical pickup and disk drive apparatus according tothe present invention will be described below with reference to theaccompanying drawings.

A disk drive apparatus 1 has members and mechanisms disposed in an outercasing 2 (see FIG. 1).

A chassis 3 is disposed in the outer casing 2, and a spindle motor, notshown, is mounted on the chassis 3. The spindle motor has a motor shaftto which there is fixed a disk table 4 projecting upwardly from thechassis 3.

A lead screw 5 and guide shafts 6, 6 are disposed parallel to each otherbelow a lower surface of the chassis 3. The chassis 3 has a placementhole 3 a defined therein, and an optical pickup 7 is disposed inplacement hole 3 a for movement radially across a disk-shaped recordingmedium 100 that is to be mounted on the disk table 4.

The optical pickup 7 has a movable base 8, an optical element (opticaldevice) mounted on the movable base 8, and an objective lens actuator 9supported on the movable base 8. The movable base 8 has opposite endsslidably supported respectively on the guide shafts 6, 6. A nut, notshown, mounted on the movable base 8 is threaded over the lead screw 5.When the lead screw 5 is rotated about its own axis by a feed motor, notshown, mounted on the movable base 8, the nut is fed in a directiondepending on the rotating direction of the lead screw 5, moving theoptical pickup 7 across the radius of the disk-shaped recording medium100 mounted on the disk table 4 while the optical pickup 7 is beingguided by the guide shafts 6, 6.

The objective lens actuator 9 has a fixed member 10 and a movable member12 movably supported on the fixed member 10 by a plurality ofsuspensions 11, 11, . . . . The movable member 12 holds an objectivelens 13 for focusing a first laser beam and a second laser beam, to bedescribed later, onto the recording surface of the disk-shaped recordingmedium 100.

Various optical elements are disposed on the movable base 8 (see FIG.2).

The optical elements include a light-emitting device 14, a grating 15, apolarization converter 16, a coupling lens 17, a light splitter 18, acollimator lens 19, the objective lens 13 in the objective lens actuator9, a cylindrical lens 20, an optical axis combiner 21, and alight-detecting device 22. These optical elements make up an opticalsystem of the disk drive apparatus 1.

The light-emitting device 14 has a first light emitter 14 a and a secondlight emitter 14 b for emitting respective laser beams having differentwavelengths. The first light emitter 14 a emits a first laser beamhaving a wavelength of 650 nm, for example, whereas the second lightemitter 14 b emits a second laser beam having a wavelength of 780 nm,for example. For recording an information signal on or reproducing aninformation signal from one disk-shaped recording medium 100, i.e., aDVD 100 a, the first light emitter 14 a emits the first laser beam. Forrecording an information signal on or reproducing an information signalfrom another disk-shaped recording medium 100, i.e., a CD 100 b, thesecond light emitter 14 b emits the second laser beam.

The first light emitter 14 a and the second light emitter 14 b arespaced from each other by a distance ranging from 30 μm to 300 μm, forexample, in a direction perpendicular to the optical axis of theobjective lens 13. For example, the first laser beam has its opticalaxis aligned with the optical axis of the objective lens 13, and thesecond laser beam has its optical axis displaced off the optical axis ofthe objective lens 13.

Conversely, the first laser beam may have its optical axis displaced offthe optical axis of the objective lens 13, and the second laser beam mayhave its optical axis aligned with the optical axis of the objectivelens 13.

The grating 15 has a function to diffract the first laser beam and thesecond laser beam to form three spots on the recording surface of thedisk-shaped recording medium 100 in order to detect a tracking errorsignal.

The polarization converter 16 comprises a ¼ wavelength plate, forexample. The polarization converter 16 has a function to limit thereturn to the light-emitting device 14 of the first laser beam and thesecond laser beam that are reflected by the disk-shaped recording medium100. The polarization converter 16 may be inclined to the optical axisto correct coma.

The coupling lens 17 is disposed in the optical path between thelight-emitting device 14 and the collimator lens 19. The coupling lens17 has a function to change the optical magnification from thelight-emitting device 14 to the disk-shaped recording medium 100 and theoptical magnification from the disk-shaped recording medium 100 to thelight-detecting device 22.

The light splitter 18 comprises a beam splitter plate, for example. Thelight splitter 18 is of the so-called transmissive type. The lightsplitter 18 passes the first and second laser beams that are emittedfrom the light-emitting device 14 toward the disk-shaped recordingmedium 100, and reflects the first and second laser beams that arereflected by the disk-shaped recording medium 100.

The light splitter 18 is not limited to a beam splitter plate, but maybe any of the transmissive type having a function to slip laser beams.For example, the light splitter 18 may comprise a beam splitter prism.

The collimator lens 19 is disposed in the optical path between the lightsplitter 18 and the objective lens 13, and has a function to convert thefirst and second laser beams into a parallel beam. The collimator lens19 may have one toric surface for correcting astigmatism.

The cylindrical lens 20 is used to detect a focusing error signalaccording to the astigmatic method. If a focusing error signal is to bedetected according to another method such as the Foucault method, ratherthan the astigmatic method, then a holographic element or a prism isused instead of the cylindrical lens 20.

The optical axis combiner 21 is an optical device with wavelengthselectivity. The optical axis combiner 21 has a function to correct thedirection of the optical axis of the second laser beam which isdisplaced off the optical axis of the objective lens 13 for applying thesecond laser beam to a predetermined light-detecting area of thelight-detecting device 22.

In the optical system thus constructed, when the first laser beam isemitted from the first light emitter 14 a of the light-emitting device14, the emitted first laser beam is diffracted by the grating 15, andapplied via the polarization converter 16 and the coupling lens 17 tothe light splitter 18. The first laser beam that has been applied to thelight splitter 18 passes through the light splitter 18, and is convertedby the collimator lens 19 into a parallel beam, which is focused ontothe recording surface of the DVD 100 a mounted on the disk table 4 bythe objective lens 13. The laser beam that has been focused onto therecording surface of the DVD 100 a is reflected thereby and applied as areturning laser beam through the objective lens 13 and the collimatorlens 19 to the light splitter 18. The returning laser beam that has beenapplied to the light splitter 18 is reflected thereby and appliedthrough the cylindrical lens 20 and the optical axis combiner 21 to thelight-detecting device 22.

When the returning laser beam is applied to the light-detecting device22 in a reproduction mode, for example, an information signal recordedon the DVD 100 a is read. At the same time, a focusing error signal anda tracking error signal are detected. Based on the detected focusingerror signal and tracking error signal, the movable member 12 of theobjective lens actuator 9 is displaced with respect to the fixed member10 for thereby making focusing and tracking adjustments.

When the second laser beam is emitted from the second light emitter 14 bof the light-emitting device 14, the emitted second laser beam isdiffracted by the grating 15, and applied via the polarization converter16 and the coupling lens 17 to the light splitter 18. The second laserbeam that has been applied to the light splitter 18 passes through thelight splitter 18, and is converted by the collimator lens 19 into aparallel beam, which is focused onto the recording surface of the CD 100b mounted on the disk table 4 by the objective lens 13. The laser beamthat has been focused onto the recording surface of the CD 100 b isreflected thereby and applied as a returning laser beam through theobjective lens 13 and the collimator lens 19 to the light splitter 18.The returning laser beam that has been applied to the light splitter 18is reflected thereby and passes through the cylindrical lens 20. Afterthe direction of the optical axis of the returning laser beam iscorrected by the optical axis combiner 21, the returning laser beam isapplied to the light-detecting device 22.

When the returning laser beam is applied to the light-detecting device22 in a reproduction mode, for example, an information signal recordedon the CD 100 b is read. At the same time, a focusing error signal and atracking error signal are detected. Based on the detected focusing errorsignal and tracking error signal, the movable member 12 of the objectivelens actuator 9 is displaced with respect to the fixed member 10 forthereby making focusing and tracking adjustments.

In the above optical system, since the optical axis of one of the laserbeams is displaced off the optical axis of the objective lens 13, thefollowing two means are employed to control the generation ofaberrations:

According to the first means, the second light emitter 14 b ispositioned such that the angle of incidence on the light splitter 18 ofthe second laser beam emitted from the light-emitting device 14 to thedisk-shaped recording medium 100 is greater than the angle of incidenceon the light splitter 18 of the first laser beam emitted from thelight-emitting device 14 to the disk-shaped recording medium 100.

The first means will be described below.

As shown in FIG. 3, if the angle of incidence of the first laser beam onthe light splitter 18 is represented by θ and the angle of incidence ofthe second laser beam on the light splitter 18 is represented by α, thensince the light splitter 18 is inclined to the optical axis, the angle αof incidence becomes smaller than the angle θ of incidence as the secondlight emitter 14 b is progressively spaced from the first light emitter14 a in one direction, and the angle α of incidence becomes greater thanthe angle θ of incidence as the second light emitter 14 b isprogressively spaced from the first light emitter 14 a in the oppositedirection. The region where the angle α of incidence is smaller than theangle θ of incidence is referred to as a positive region, and the regionwhere the angle α of incidence is greater than the angle θ of incidenceis referred to as a negative region.

FIGS. 4 through 8 are graphs showing amounts of aberrations generatedwith respect to the distance from the second light emitter 14 b to thefirst light emitter 14 a (hereinafter referred to as “inter-emitterdistance”) at the time the distance D (see FIG. 3) from thelight-emitting device 14 (the first light emitter 14 a, the second lightemitter 14 b) to the coupling lens 17 is changed with respect to thecombined focal length F (see FIG. 3) of the collimator lens 19 and thecoupling lens 17. The inter-emitter distance is negative in the regionwhere the angle α of incidence is greater than the angle θ of incidenceand positive in the region the angle α of incidence is smaller than theangle θ of incidence. In the graphs, the magnitudes of the aberrationsare represented by a predetermined coefficient (Fringe Zernikecoefficient).

As shown in the graphs of FIGS. 4 through 8, the amount of generatedspherical aberration remains substantially unchanged even if theinter-emitter distance is changed. The amount of generated coma is 0 ifthe inter-emitter distance is 0 or negative. The amount of generatedastigmatism is 0 if the inter-emitter distance is negative. Therefore,in order to suppress the generation of the aberrations, it is desirableto place the second light emitter 14 b at a position where theinter-emitter distance is negative, i.e., the angle α of incidence isgreater than the angle θ of incidence.

In the above optical system, therefore, the second light emitter 14 b isplaced at a position where the angle α of incidence is greater than theangle θ of incidence. For example, the second light emitter 14 b isdisposed in a position that is 0.2 mm spaced from the first lightemitter 14 a in the negative region.

According to the second means, the light-emitting device 14, thecoupling lens 17, and the collimator lens 19 are disposed in a positionwhere the distance D from the light-emitting device 14 to the couplinglens 17 is 0.26F or more with respect to the combined focal length F ofthe collimator lens 19 and the coupling lens 17.

The second means will be described below.

As shown in the graphs of FIGS. 4 through 8, the amount of generatedspherical aberration does not greatly increase or decrease even if thedistance D is changed with respect to the focal length F. The amount ofgenerated coma is small at D=0.14F, D=0.26F, and D=0.38F, and falls in arelatively small range at D=0.51F and D=0.64F. The amount of generatedastigmatism is particularly greatly changed depending on theinter-emitter distance at D=0.14F. Therefore, it is desirable to havethe distance D of 0.26F or greater in order to suppress the generationof the aberrations.

In the above optical system, the light-emitting device 14, the couplinglens 17, and the collimator lens 19 are disposed such that the distanceD is of 0.26F or greater.

As described above, in order to suppress the generation of theaberrations, it is desirable to place the second light emitter 14 b at aposition where the angle α of incidence is greater than the angle θ ofincidence, and to place the light-emitting device 14, the coupling lens17, and the collimator lens 19 such that the distance D is of 0.26F orgreater. In particular, it is most desirable to minimize the totalamount of generated spherical aberration, coma, and astigmatism. Forexample, if the distance D is of about 0.38F and the inter-emitterdistance is of about 0.2 in the negative region, then the total amountof the generated aberrations is small, and hence the generation of theaberrations can effectively be suppressed.

As described above, in the optical pickup 7, the second light emitter 14b is placed at a position where the angle α of incidence of the secondlaser beam on the light splitter 18 is greater than the angle θ ofincidence of the first laser beam on the light splitter 18.

Therefore, the generation of the aberrations is suppressed. The opticalpickup 7 can have better performance, prevent recording errors andreproduction errors, and provide improved playability.

Furthermore, in the optical pickup 7, the distance D from thelight-emitting device 14 to the coupling lens 17 is of 0.26D or morewith respect to the combined focal length F of the collimator lens 19and the coupling lens 17.

Consequently, the generation of the aberrations is further suppressed.The optical pickup 7 can have better performance, reliably preventrecording errors and reproduction errors, and provide further improvedplayability.

In the above embodiment, the first laser beam emitted from the firstlight emitter 14 a has its optical axis aligned with the optical axis ofthe objective lens 13, and the second laser beam emitted from the secondlight emitter 14 b has its optical axis displaced off the optical axisof the objective lens 13. Conversely, if the second laser beam has itsoptical axis aligned with the optical axis of the objective lens 13, andthe first laser beam has its optical axis displaced off the optical axisof the objective lens 13, then the distance D represents a distance upto the first light emitter 14 a using the second light emitter 14 b as areference.

The specific shapes and structures of the various parts illustrated inthe above embodiment are given by way of example only in reducing thepresent invention to practice, and should not be relied upon in any wayto lead to a restrictive interpretation of the technical scope of thepresent invention.

INDUSTRIAL APPLICABILITY

As is apparent from the foregoing description, according to theinvention as described in claim 1, claim 2, claim 3, and claim 4, thegeneration of the aberrations is suppressed, and the optical pickup canhave better performance, prevent recording errors and reproductionerrors, and provide improved playability.

1. An optical pickup movable radially across a disk-shaped recordingmedium mounted on a disk table, for applying a laser beam to thedisk-shaped recording medium through an objective lens, comprising: alight-emitting device having a first light emitter and a second lightemitter for emitting, to two different types of disk-shaped recordingmedia, respectively, a first laser beam and a second laser beam,respectively, having a first wavelength and a second wavelength,respectively, which correspond to the respective disk-shaped recordingmedia; a light splitter for passing the first laser beam and the secondlaser beam which are emitted from the light-emitting device toward thedisk-shaped recording media, and reflecting the first laser beam and thesecond laser beam which are reflected respectively by the disk-shapedrecording media; a collimator lens disposed between the light splitterand the objective lens, for converting the first laser beam and thesecond laser beam which are emitted from the light-emitting device intoa parallel beam; a light-detecting device for being irradiated with thefirst laser beam and the second laser beam which are reflected by thelight splitter; and a coupling lens disposed between the light-emittingdevice and the collimator lens, for changing an optical magnificationfrom the light-emitting device to the disk-shaped recording media and anoptical magnification from the disk-shaped recording media to thelight-detecting device; the first laser beam having an optical axisaligned with the optical axis of the objective lens, the second laserbeam having an optical axis displaced off the optical axis of theobjective lens; wherein the distance from the light-emitting device tothe coupling lens is of 0.26F or more where F represents the combinedfocal length of the collimator lens and the coupling lens.
 2. A diskdrive apparatus having a rotatable disk table for mounting a disk-shapedrecording medium thereon, and an optical pickup having an objective lensactuator supported on a movable base and movable radially across thedisk-shaped recording medium mounted on the disk table, for applying alaser beam to the disk-shaped recording medium through an objectivelens, wherein said optical pickup comprising: a light-emitting devicehaving a first light emitter and a second light emitter for emitting, totwo different types of disk-shaped recording media, respectively, afirst laser beam and a second laser beam, respectively, having a firstwavelength and a second wavelength, respectively, which correspond tothe respective disk-shaped recording media; a light splitter for passingthe first laser beam and the second laser beam which are emitted fromthe light-emitting device toward the disk-shaped recording media, andreflecting the first laser beam and the second laser beam which arereflected respectively by the disk-shaped recording media; a collimatorlens disposed between the light splitter and the objective lens, forconverting the first laser beam and the second laser beam which areemitted from the light-emitting device into a parallel beam; alight-detecting device for being irradiated with the first laser beamand the second laser beam which are reflected by the light splitter; anda coupling lens disposed between the light-emitting device and thecollimator lens, for changing an optical magnification from thelight-emitting device to the disk-shaped recording media and an opticalmagnification from the disk-shaped recording media to thelight-detecting device; the first laser beam having an optical axisaligned with the optical axis of the objective lens, the second laserbeam having an optical axis displaced off the optical axis of theobjective lens; wherein the distance from the light-emitting device tothe coupling lens is of 0.26F or more where F represents the combinedfocal length of the collimator lens and the coupling lens.